A PV curve (PV loop) is run through during the artificial respiration of intubated patients who are not breathing spontaneously and this is used to measure the lung. For example, a volume that is usually substantially larger than the maximum volume normally applied during the current respiration is applied in this case. This volume is subsequently expired again. The volume and the pressure are recorded during the inspiration and expiration in an X-Y plot (curve). The so-called inflection points (opening and closing points) of the lungs, the so-called lung compliance at different pressures and other measured variables can then be determined from this curve in order to then optimize the respiration pressures of the current respiration.
A process of the type described in the introduction is known, for example, from the document WO 03/037413 A1. A prolonged phase of expiration is carried out in the process described there before the PV manoeuver in order to lower the pressure to a preset minimum value, which is the so-called positive end expiratory pressure (PEEP). The inspiration pressure is then increased continuously over a pressure ramp to a preset peak pressure and the volume is determined at the same time, which is possible, e.g., by detecting the breathing gas volume flow and by integrating same. A passive expiration to the end pressure is carried out after the manoeuver described. The pressure curve is then plotted as a two-dimensional curve against the volume. The PV curve can then be analyzed in the known manner, and the normal breathing cycles can be continued with the respirator. Furthermore, the possibility of allowing the pressure to drop from the preset maximum over a pressure ramp to the desired minimum in a controlled manner is addressed. Based on the nonlinear flow behavior of the lungs, a more or less greatly varying breathing gas volume flow with peaks is now obtained.
One problem in the prior-art PV measurement maneuvers is that they ignore the fact that a pressure drop is also brought about by flow resistances of the breathing gases in the airways and in the lungs. However, these effects on the PV curve obtained, which are due to the flow resistance, cannot be ignored any longer in case of larger breathing gas volume flows. Even though it would be possible to avoid this problem by running through a slow pressure rise ramp and a correspondingly slow expiration curve, this would require an excessively long measuring time and unduly stress the patient.